Steel which contains boron is usually used for producing high tensile strength steel at a low cost. Its hardenability is a factor which has an important bearing on the strength and toughness of a product obtained by quenching and tempering.
Boron is added to steel for the sole purpose of improving its hardenability. Large quantities of aluminum and titanium are always added, too, in order to eliminate undesirable effects of nitrogen on boron so that the boron may be fully effective. The addition of aluminum and titanium also has a grain-refining effect. It is usually necessary and sufficient to add boron in such a quantity that steel may contain 5 to 20 ppm of acid-soluble boron.
The production and use of boron steel, however, involve the following disadvantages:
(1) In the event a steel wire having a tensile strength of 150 kg/mm.sup.2 is produced by oil tempering, the ductility of the product is not very high immediately after heat treatment, but is recovered to a prescribed level with the lapse of time. If the product is meanwhile under stress, "delayed fracture" is likely to occur.
(2) When a continuously cast billet is hot rolled, it is likely to crack along an oscillation mark. This is particularly likely when a hot continuously cast billet is reheated and rolled.
(3) Boron steel is difficult to be cast continuously, since aluminum, which is essential for boron steel, is likely to close a tundish nozzle. Titanium has a similar action, but if its quantity is small, heavy corrosion of nozzle refractories takes place, and renders continuous-continuous casting difficult.